Add an external link marker to external links

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>

layouts/_default/_markup/render-link.html

@@ -0,0 +1,13 @@
+{{- $absoluteDest := absLangURL .Destination -}}
+{{- $siteRootUrl := absLangURL "" -}}
+{{- $isExternal := not (hasPrefix $absoluteDest $siteRootUrl) -}}
+<a href="{{ .Destination | safeURL }}"
+  {{- with .Title }} title="{{ . }}"{{ end -}}
+  {{- if $isExternal -}}
+   class="external-link" target="_blank" rel="noopener noreferrer"
+  {{- end -}}
+>
+  {{- with .Text | safeHTML }}{{ . }}{{ end -}}
+  {{- if $isExternal -}}{{- partial "icon.html" "external-link" -}}{{- end -}}
+</a>
+{{- /* chomp trailing newline */ -}}